Rhodamne Essay

The absorption band for the popular rhodamine – 6G dye is about ~ 100 nm at full width half maximum (FWHM) wide and the fluorescent emission band is about ~ 150 nm wide around the centre wavelength, which shows nearly 50 nm Stokes shift.

Rhodamine – 6G exhibits a near unity quantum efficiency and may lose its efficiency, if the dye concentration is too high, due to interaction between the dye molecules. The efficiency of rhodamine – 6G is significantly reduced when highly polar solvents such as water, ethanol, methanol and ethylene glycol are used. The absorption (A) of pump light from the electronic singlet ground state S0 to the first excited singlet state S1 is followed by a rapid thermalization in the excited vibronic manifold in S1.

As the fluorescence decay time for the dye molecule is a few nanoseconds, an intense pumping source is required to achieve significant population inversion in a fairly short time. The inversion must be sufficient to produce a net gain in the laser to overcome the absorption losses. The intensities required for a typical dye laser are of the order of 1 MW/cm2. The excited molecule can lose its energy by emission of a photon (F), undergo a non-radiative transition to the ground state or can populate the triplet state via intersystem crossing (kST). An energy level diagram of a typical organic dye is shown in figure 1.2. Figure 1.2 shows that there are two energy manifold in dye molecules: the singlet (Sn) and triplet (Tn) states. The excited triplet states are long lived in comparison with the excited singlet sates. The internal conversion (transitions in between states of same multiplicity) and intersystem crossing (kST) reduce the quantum efficiency for fluorescence. Once the triplet state is populated, this may lead to further losses of dye molecules by internal triplet absorption as shown in figure 1.2. The triplet absorption cross section T